1. Field of the Invention
The present invention relates to a sealing material made of expanded graphite which is used in, for example, various kinds of sealing members for high-temperature use such as a packing, a gasket, a V-shaped ring, a valve seat, and a sliding material such as a bearing, or a heat insulating material for a high temperature vacuum furnace. The present invention relates also to a method of producing the sealing material, and to a gasket sheet.
2. Description of the Prior art
Generally, rubber or a polytetrafluoroethylene resin (PTFE, the trade name is Teflon) has been used in various types of sealing members for high-temperature use. In recent years, sealing members made of expanded graphite which is superior in terms of heat resistance and the like have widely been developed and often used.
Expanded graphite is obtained by performing an expansion process on acid-treated graphite 1A having a thickness of H0 and a laminate structure of flaky graphite particles 1a shown in FIG. 22. The expanded graphite consists as shown in FIG. 23 of a bellow-like expanded graphite structure 1 having a thickness H (about 5 to 10 mm) in which laminations of graphite particles 1a are opened in a laminate direction (a direction indicated by arrow a) so that a gap G is formed between the graphite particles 1a.
Such an expanded graphite structure 1 is used in, for example, a gasket sheet, a die-molded product of a sheet, a string-like material combined with a fiber, or a material obtained by braiding such materials.
As a gasket sheet, for example, known is a laminate processed article which is obtained in the following manner. Press molding or roll molding is performed for applying a pressure to the expanded graphite structure 1, so that the gaps G in the opened bellow-like structure 1 which has been described with reference to FIG. 23 is reduced or eliminated. Thus, the graphite particles 1a are again in contact with each other, thereby forming a sheet-like base member 201 as shown in FIG. 24 in which the graphite particles la are self-bonded to each other. Laminate members 202 and 202 made of a PTFE film are bonded to the upper and lower faces of the sheet-like base member 201 with a binder, respectively.
In addition, known materials include a sheet-like material obtained by applying a pressure to the expanded graphite structure in which a binder is mixed, by means of press molding or roll-pressurizing molding, and a material obtained by forming local embossed portions on the principal face of a sheet-like base member made of an expanded graphite structure by using an embossing roll or other tools, and then by bonding a foil or the like with using a laminate process, or by coating a rubber material.
In the gasket sheet made of a conventional expanded graphite sealing material having the above-described structure, the sheet-like base member 201 is obtained by simply applying a pressure to the expanded graphite structure 1, so that the graphite particles 1a on the principal face side of the base member 201 are in a high orientation state with high density. As shown in FIG. 25, therefore, the crystal planes of the portion are closely in contact with each other under pressure substantially in parallel to the principal face.
Such a high orientation state exhibits a poor surface bonding property. In the case where a laminate member 202 made of a PTFE film or the like is to be bonded to the base member by a laminate process, it is difficult for the laminate member 202 of the PTFE film as shown in FIG. 24 or the binder to penetrate between the expanded graphite particles 1a1 in the high orientation portion on the principal face side. This may easily cause the peeling of the laminate member 202 or the binder layer.
In addition, the above-described process for simply applying a pressure to the expanded graphite structure 1 cannot overcome inherent drawbacks of the expanded graphite structure 1 in that the airtight sealing property is poor (air leakage), the strength is low, and the principal face of the base member 201 is easily damaged.
If the gasket sheet is used in a sandwiched state between joint flanges, there arise further problems as follows. The laminate member 202 may be stuck to the joint flange face, or the components such as sulfur, and chlorine contained in the graphite particles 1a may attack and corrode the counterpart metal.